In a semiconductor avalanche photodetector, the noise depends in part upon the ratio of the ionization rates of the two types of charge carriers present in semiconductor devices and on the mechanism which initiates carrier multiplication. The ionization rate is the average number of ionizing collisions a carrier undergoes per unit path length traveled. A large difference between the ionization rates of the electrons and holes results in desirable noise characteristics for an avalanche photodetector. Preferably, the avalanche is initiated by the charge carrier having the larger ionization rate, however significant reduction in noise can be achieved as long as the smaller ionization rate is negligible compared to the larger ionization rate. In the ideal avalanche detector, only one type of charge carrier would undergo ionizing collisions. (See McIntyre, R. J., IEEE Transactions on Electron Devices, Vol. 13, p. 194 (1966).)
In contrast with silicon, many group III-V semiconductor materials exhibit ionization rates for electrons and holes which do not significantly differ. Accordingly, efforts have been directed to tailoring the structure of an avalanche photodetector comprising group III-V or other semiconductors so that substantial noise reduction is achieved in accordance with the above-described physical principles.